This application claims priority to Republic of Korea patent application 2000-68493 filed Nov. 17, 2000.
1. Field of the Invention
The present invention relates, in general, to a dielectric ceramic composition and, more particularly, to a dielectric ceramic composition controllable in temperature coefficient of resonant frequency and sinterable at low temperatures, suitable for use in planar or multilayer type electronic parts containing inner conductors, for the fabrication of which simultaneously sintering base materials and conductors is required. Also, the present invention is concerned with a method for manufacturing such a dielectric ceramic composition.
2. Description of the Prior Art
With great advances in electronic and communication technologies, apparatuses and equipments for embodying them have recently been miniaturized. To this miniaturization, stacking and chipping techniques of electronic parts make a great contribution. Generally, ceramic materials for use in electronic parts are divided into dielectrics and magnetics. Recently, particular pressure has been placed on electronic parts made of dielectrics to miniaturize.
Representative of the electronic parts to which stacking techniques are applied is a capacitor. Examples of multilayer type electronic parts for use in mobile communication terminals include filters, couplers, duplexers, oscillators and multichip modules (MCM). The multilayer type electronic parts, most portions of which are formed of multiplayer dielectrics and inner electrodes, are fabricated by laminating a dielectric into a tape, printing an inner electrode onto the dielectric laminate, stacking a plurality of laminates and firing the stack.
To be useful for multilayer type elements, accordingly, dielectrics must be capable of being sintered along with electrodes in addition to having dielectric properties suitable for application. Such dielectric requirements include high dielectric constant, low dielectric loss, and low dependency of resonant frequency modulation on temperature change and the like.
Materials suitable for the inner electrodes are silver, copper, nickel, palladium, platinum, gold and alloys thereof. Selection of one of the inner electrode materials is made depending on the sintering temperature and properties of the ceramic dielectric used and vice versa.
For example, silver (Ag), showing the lowest specific resistance (1.62xc3x9710xe2x88x924 xcexa9cm) and being inexpensive, cannot be applied to ceramic dielectrics which must be sintered at 950xc2x0 C. or higher because of its low melting point (961xc2x0 C.). In spite of their high melting points, gold (Au), platinum and palladium (Pd) are restricted in their use because of high price. As for copper (Cu) or nickel (Ni) electrodes, their very poor oxidation resistance requires sintering at an oxygen partial pressure as low as about 10xe2x88x929 atm, causing the problem that, when thermally treated under such a low oxygen partial pressure, most dielectric ceramic compositions show highly increased dielectric loss and thus cannot be used as capacitors.
Ceramic dielectric compositions currently used in multilayer type electronic parts are, for the most part, based on BaTiO3, optionally added with oxide sintering aids or glass frits for reduction of sintering temperatures. Typically, these dielectric compositions range, in sintering temperature, from 1,100 to 1,300xc2x0 C., as well as being resistant to reduction and having dielectric constants of several hundreds or higher. However, their great dielectric loss makes it difficult to apply them to the products which are used for high frequency band of MHz or higher. Additionally, the dielectric compositions suffer from the drawback of undergoing a dielectric constant fluctuation of as large as hundreds ppm/xc2x0 C., which prevents them from being applied to temperature-stable capacitors or electronic parts for mobile communication.
Dielectric compositions known to be usable for multilayer type elements operable with frequencies of MHz or higher are exemplified by Cuo or V2O5-added BiNbO4 and glass-added (Mg, Ca)TiO3, (Zr, Sn)TiO4 or (BaOxe2x80x94TiO2xe2x80x94WO3). These compositions, however, have drawbacks of being not effectively sintered at a temperature lower than 1,000xc2x0 C., being poor in dielectric properties at microwave frequencies, and showing large reactivity with electrode materials.
Another well-known dielectric composition is based on a BaOxe2x80x94TiO2xe2x80x94Nb2O5 system, whose subtypes comprise BaTiNb4O13, Ba3Ti4Nb4O21 and Ba3Ti5Nb6O28 (see. xe2x80x9cNew low microwave dielectric ceramics in the BaOxe2x80x94TiO2xe2x80x94Nb2O5/Ta2O5 systemxe2x80x9d, M. T. Sebastian, Journal of Materials Science; Materials in Electronics, Vol. 10 (1999), pp. 475-478). U.S. Pat. No. 4,767,732 discloses a high dielectric constant ceramic material having a high dielectric constant and a high insulation resistance obtained by mixing BaTiO3 powder and oxides of Pb, Ba, Sr, Zn, Nb, Mg and Ti. These compositions Ba3Ti4Nb4O21 and BaTiO3, in spite of having a dielectric constant and a quality factor (Qxc3x97f) amounting to as large as 55 and 9,500, respectively, is virtually impossible to apply to electronic parts which require high frequencies of microwave bands or high temperature stability for their operation because its temperature coefficient of resonant frequency is 100 ppm/xc2x0 C. Furthermore, this publication does not describe low-temperature sintering properties of the composition.
With the problems encountered in prior arts in mind, the present invention has an object of providing a dielectric ceramic composition, which is controllable in temperature coefficient of resonant frequency and sinterable at low temperatures.
It is another object of the present invention to provide a dielectric ceramic composition with excellent dielectric properties suitable for use in planar or multilayer type elements operable with frequencies from MHz to GHz.
It is a further object of the present invention to provide a method for manufacturing such a dielectric ceramic composition.
Based on the present invention, there is provided a ceramic Composition comprising as a main component a compositions represented by
(1xe2x88x92X)Ba3Ti4Nb4O21xe2x88x92(X)ANb2O6
wherein, X is a mole fraction between 0 and 1; and A is Ba or Sr.
A is Ba or Sr.
According to one aspect of the invention, the ceramic composition may further comprise at least one oxide selected from the group consisting of B2O3, CuO and ZnO, and or alternatively at least one additive selected from the group consisting of V2O5, SnO2, MgO, NiO, Sb2O3, Bi2O3 and Ag2O.
The present invention further provides a method for manufacturing a dielectric ceramic, comprising the steps of: mixing oxide powders to give a composition composed of (1xe2x80x94x)Ba3Ti4Nb4O21xe2x80x94(x)Ba(or Sr)Nb2O6; drying and calcining the composition; milling the calcinated composition; molding the milled composition to give a molded body; and sintering the molded body.
In another aspect of the present invention, the process may further comprise the steps of adding the sintering aids and/or additive to the composition before calcination.
The present invention contemplates a composite dielectric ceramic composition suitable for use in microwave bands, comprising two separate ingredients with different temperature coefficients of resonant frequency, one being of negative temperature coefficient of resonant frequency, the other of positive. As a dielectric ceramic with a negative dielectric constant, Ba3Ti4Nb4O21 is used in the present invention. The other ceramic useful in the present invention is composed of BaNb2O6 or SrNb2O6. Optionally, the composite dielectric ceramic composition comprises a sintering aid selected from the group consisting of B2O3, CuO and ZnO.
A temperature of as high as 1,200xc2x0 C. is needed to sinter the Ba3Ti4Nb4O21xe2x80x94Ba(or Sr)Nb2O6-based composition (sometimes, xe2x80x9cbase compositionxe2x80x9d), basically excluding the possibility of sintering a low-melting point metal electrode, such as silver, copper or silver/palladium alloy, together. The sintering aid acts to reduce the sintering temperature of the base composition to 900-1,000xc2x0 C., making it possible to use such a low-melting point metal electrode.
A preferable amount of the sintering aid falls within the range of 0.01-7 weight parts of the base composition. For example, when the sintering aid is used at such an amount, the sintering can be facilitated and the composition can be improved in dielectric properties. On the other hand, when the amount of the sintering aid is outside the preferred range, no improvements can be expected in either the sintering or the dielectric properties of the composition.
Besides the sintering aid, the Ba3Ti4Nb4O21xe2x80x94Ba(or Sr)Nb2O6-based composition according to the present invention may comprise at least one oxide additive selected from the group consisting of V2O5, SnO2, MgO, NiO, Sb2O3, Bi2O3 and Ag2O. This oxide additive is preferably added in an amount of 0.01-7 weight parts of the base composition. Within this range, the additive acts to improve sintering properties of the base composition.
To improve dielectric properties, the Ba3Ti4Nb4O21 moiety of the base composition may be partially substituted with cationic substituents such as SnO2, ZrO2 or Ta2O5. The presence of such cationic substituents in the Ba3Ti4Nb4O21 moiety brings about advantages of reducing the dielectric loss and modulating the temperature coefficient of resonant frequency. When the TiO2 of the Ba3Ti4Nb4O21 moiety is partially substituted with SnO2 or ZrO2 a preferable amount of these substituent oxides fall within the range of 0.01-50 mol %. For instance, if too much or too little SnO2 or ZrO2 is added, it can cause an increase in either the dielectric loss or the temperature coefficient of resonant frequency. Ta2O5 is used to substitute Ta for the Nb of the Ba3Ti4Nb4O21. The substituting amount of Ta is preferably within the range of 0.01-50 mol %. If the amount of Ta is outside of this range, deterioration is found in the sintering properties of the composition.
Below, a description will be given of the manufacturing process of the ceramic dielectrics according to the present invention.
The composition of (1xe2x88x92x)Ba3Ti4Nb4O21xe2x88x92(x)Ba(or Sr)Nb2O6 may be prepared from materials composed of Ba3Ti4Nb4O21 or its cation-substituted material, and materials composed of Ba(or Sr)Nb2O6.
Thus, a material composed of Ba3Ti4Nb4O21 or its cation-substituted material, and a material composed of Ba(or Sr)Nb2O6 are separately prepared, followed by mixing the two compositions at an appropriate mole ratio to give a base composition with a suitable temperature coefficient of resonant frequency. By virtue of its superior microwave characteristics, the base composition can be directly applied to multilayer type electronic parts for high frequencies.
Optionally, the base composition may further comprise sintering aids and/or additives in order to improve its low-temperature sintering properties.
To the Ba3Ti4Nb4O21xe2x80x94Ba(or Sr)Nb2O6 -based composition is added at least one oxide selected from the group consisting of B2O3, CuO and ZnO, as sintering aid in an amount described above. Otherwise, the oxides can be added prior to or during the step of mixing the material composed of Ba3Ti4Nb4O21 or its cation-substituted material and the material composed of Ba(or Sr)Nb2O6.
Thus obtained mixture is demoisturized. After calcination, the demoisturized mixture is milled. Addition of a binder enables the resulting powder to be molded and sintered to yield a dielectric. During this procedure, the oxide serves as a sintering aid for improving the low-temperature sintering properties.
Besides the sintering aid, the composition may further comprises at least one additive selected from the group consisting of V2O5, SnO2, MgO, NiO, Bi2O3, Ag2O and Sb2O3 for similar purposes, which may be added prior to, during or after the mixing step.